You will not pass through UK customs
You will not pass through UK immigration
Your bags will be checked through to your final BA destination, so won't have to be collected or re-checked. (Check the baggage tags given to you at checkin to ensure they have been tagged all the way through)
If your starting airport is able, they'll give you your second boarding pass
If not, you'll have to go to the flight connections desk to collect it
On arrival, you'll follow the purple Flight Connections signs, and have your boarding pass + passport checked
You will have to clear security, which is upstairs from flight connections, before you enter the main departures area of the terminal
Ethan Stark